There's no arguing with success, and no doubt Clark's eager following will lap up her 13th romantic suspense novel as eagerly as ever. All the elements are in place: an appealing, plucky working-girl heroine placed in instant danger; a virile, adoring would-be lover kept at arm's length until the curtain; a cute moppet (also in danger); a doting but somewhat foolish mother; a dead dad whose spirit is ever-present in times of crisis. What's lacking is any real suspense, or, in this case, a satisfactory windup. Lacey Farrell is a comely young real estate saleswoman in Manhattan who has a client, Isabelle Waring, murdered virtually before her eyes, then has to spend most of the book on the run from the killer, whom only she can identify. In the process she goes into the witness protection program, and the most interesting part of the novel (Clark is always good on research) is the details on how this works. The plot, however--involving Isabelle's certainty that her daughter was murdered, the suspicion that falls on the wealthy man who owns Lacey's real estate firm and his scapegrace son, and a hit man who remorselessly pursues Lacey--is perfunctory in the extreme. When the real villain is finally unmasked--in a few throwaway sentences--the reader has almost forgotten he existed and is given no clue as to how and why he did all his evil deeds. Maybe 13 isn't Clark's lucky number. 1.1 million first printing; Literary Guild main selection; author tour. (May)